This invention relates to an access manhole that is mounted in a storage or process tank or the like and, in particular, to a method of replacing an outwardly opening manhole cover with an inwardly opening manhole cover.
Many storage or process tanks and, in particular, older tanks, are equipped with manholes that include covers that open outwardly with regard to the tank. Typically, the cover is bolted to a flange that encircles the outside of a cylindrical manway. The term manway, as herein used, refers to a tubular member that is welded or otherwise joined to the tank to provide access into the tank. In the event the tank is pressurized or contains material that exerts an outwardly directed force against the cover, the bolts holding the cover in place must be strong enough to prevent unwanted opening of the cover. By replacing the outwardly opening cover with one that opens inwardly, any material forces that are exerted against the cover will help to hold the cover closed and will prevent material stored in the tank from collecting in the manway. Material that is collected in the manway produces unwanted spillage when the outwardly opening cover is removed and under certain conditions, can be hazardous. In addition, in process vessels, material which collects in the manway does not circulate freely within the vessel and thus, is typically over processed.
Replacing an outwardly opening manhole within a storage tank with a more preferred inwardly opening cover also has heretofore presented certain structural difficulties where the existing manway is removed from the tank and replaced with an entirely new manhole unit. Complete removal of the existing manhole can produce structural damage to the tank shell in and about the manway opening that oftentimes requires extensive repair to the tank. Furthermore, retrofitting a new manway to the repaired opening is sometimes difficult and can result in damage to existing tank coatings, linings or insulation.